Abstract

Just as a soggy paper straw is prone to yielding under the applied suction of a thirsty drinker, the xylem tracheids in leavesseem prone to collapse as water potential declines, impeding their function. Here we describe the collapse, under tension, oflignified cells peripheral to the leaf vein of a broad-leaved rainforest conifer, Podocarpus grayi de Laub. Leaves of Podocarpusare characterized by an array of cylindrical tracheids aligned perpendicular to the leaf vein, apparently involved in thedistribution of water radially through the mesophyll. During leaf desiccation the majority of these tracheids collapsed fromcircular to flat over the water potential range 21.5 to 22.8 MPa. An increase in the percentage of tracheids collapsed duringimposed water stress was mirrored by declining leaf hydraulic conductivity (Kleaf), implying a direct effect on water transportefficiency. Stomata responded to water stress by closing at 22.0 MPa when 45% of cells were collapsed and Kleaf had declinedby 25%. This was still substantially before the initial indications of cavitation-induced loss of hydraulic conductance in the leafvein, at 23 MPa. Plants droughted until 49% of tracheids had collapsed were found to fully recover tracheid shape and leaffunction 1 week after rewatering. A simple mechanical model of tracheid collapse, derived from the theoretical bucklingpressure for pipes, accurately predicted the collapse dynamics observed in P. grayi, substantiating estimates of cell wallelasticity and measured leaf water potential. The possible adaptive advantages of collapsible vascular tissue are discussed.